In recent years, multiple-input multiple-output (MIMO) transmission systems for performing spatial multiplexing transmission using multiple transmitting and receiving antennas have gained much attention in wireless communication. In addition, in MIMO transmission, precoding techniques have been introduced in order to improve performance of the reception. Precoding is a process performed by a transmitter side for generating transmission signals to be transmitted from multiple transmitting antennas in such a manner that the transmission signals are easily identified on a receiver side. In this case, precoding is implemented by performing linear processing in which original transmission signals are multiplied by precoding matrices.
Regarding the conventional technology, precoding technology has been proposed in which a downlink overhead is reduced (see, for example, Japanese Laid-open Patent Publication No. 2009-4921). In addition, another proposed precoding technology is to provide a minimum bit error rate in the case where most likelihood detection is used (see, for example, Japanese Laid-open Patent Publication No. 2007-110664).
An optimal precoding matrix to be used in the multiplication of a transmission signal changes depending on the condition of a channel (propagation channel) between the transmitter and the receiver. For this reason, the transmitter prepares multiple different precoding matrices in advance, and receives, from the receiver side, feedback notification of an index value which indicates a precoding matrix to be used according to the channel condition. The index value pertaining to the precoding matrix is called a precoding matrix indicator (PMI).
Conventional PMI feedback control is described here by taking, as an example, a wireless base station and a mobile station to which a high-speed wireless communications system, Long Term Evolution (LTE), is applied. In the LTE technology, orthogonal frequency division multiplexing (OFDM) is adopted as a downlink (i.e., from the wireless base station to the mobile station) modulation scheme. FIG. 16 illustrates a signal format. The minimum unit of the signal is a subcarrier, and transmission data is allocated to each subcarrier. For example, in the case of transmitting data using a modulation scheme of quadrature phase shift keying (QPSK), 2-bit data is allocated to each subcarrier.
A signal unit formed by integrating multiple successive subcarriers is a resource block (hereinafter, sometimes referred also to as “RB”). Further, a signal unit formed by integrating multiple successive resource blocks is a subband. In the LTE technology, precoding is implemented using a resource block as the minimum unit. Note that in the following description, a subband is sometimes referred to as a predetermined frequency band, and a frequency band corresponding to a resource block is sometimes referred to as a sub-frequency band.
In the LTE technology, a mode is provided in which the mobile station selects a PMI with respect to each subband, which includes multiple successive resource blocks, and the selected PMI is fed back to the wireless base station (see, for example, TR36.213 V8.5.0 in 3GPP LTE specification documents). FIG. 17 illustrates an example of PMI feedback control. Assume that each subband includes, for example, four resource blocks. A subband b1 includes resource blocks r11 to r14, a subband b2 includes resource blocks r21 to r24, and a subband b3 includes resource blocks r31 to r34.
An arrow in each of the resource blocks in FIG. 17 illustrates an image of an ideal precoding matrix for the corresponding resource block. A precoding matrix m11 is used for precoding of the resource block r11, and a precoding matrix m12 is used for precoding of the resource block r12. Similarly, a precoding matrix m13 is used for precoding of the resource block r13, and a precoding matrix m14 is used for precoding of the resource block r14. In a similar fashion, precoding matrices m21 to m24 are used for precoding of the resource blocks r21 to r24, respectively. Precoding matrices m31 to m34 are used for precoding of the resource blocks r31 to r34, respectively.
The mobile station selects an optimum precoding matrix with respect to each subband according to, for example, the channel condition, and feeds a PMI corresponding to the selected precoding matrix back to the wireless base station. For example, for the subband b1, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M1 to be applied to the resource blocks r11 to r14. The PMI corresponding to the precoding matrix M1 is referred to as “PMI#1” in FIG. 17. Similarly, for the subband b2, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M2 to be applied to the resource blocks r21 to r24. The PMI corresponding to the precoding matrix M2 is referred to as “PMI#2” in FIG. 17. Further, for the subband b3, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M3 to be applied to the resource blocks r31 to r34. The PMI corresponding to the precoding matrix M3 is referred to as “PMI#3” in FIG. 17. Subsequently, based on each fed-back PMI, the wireless base station recognizes a precoding matrix to be used, and performs precoding by multiplying a transmission signal in a corresponding subband by the precoding matrix.
In the case where the delay spread of the channel between the mobile station and the wireless base station is small and there is little channel variation in the frequency direction, the variation in the precoding matrices of the resource blocks in each subband becomes small. According to FIG. 17, in each subband, the arrows of the precoding matrices point in the same direction. This indicates that there is a small variation in the precoding matrices within each subband. Therefore, in the above-described case with the conventional precoding, it is less likely that there is a large dissimilarity between an ideal precoding matrix for each resource block in a subband and a precoding matrix indicated by a PMI which is selected by the mobile station for the subband and fed back to the wireless base station. Accordingly, the wireless base station is able to achieve the effect of precoding by performing precoding using each precoding matrix indicated by a PMI which is fed back from the mobile station.
However, in the case where the delay spread of the channel between the mobile station and the wireless base station is large, the channel variation in the frequency direction becomes large. If the above-mentioned feedback control is performed under such a channel condition, the precoding effect is not achieved and thus the reception performance is degraded. FIG. 18 illustrates another example of PMI feedback control. A precoding matrix m11a is used for precoding of the resource block r11, and a precoding matrix m12a is used for precoding of the resource block r12. Similarly, a precoding matrix m13a is used for precoding of the resource block r13, and a precoding matrix m14a is used for precoding of the resource block r14. In a similar fashion, precoding matrices m21a to m24a are used for precoding of the resource blocks r21 to r24, respectively. Precoding matrices m31a to m34a are used for precoding of the resource blocks r31 to r34, respectively.
In the case where the mobile station performs PMI feedback to the wireless base station, for the subband b1, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M1a to be applied to the resource blocks r11 to r14. The PMI corresponding to the precoding matrix M1a is referred to as “PMI#1a” in FIG. 18. Similarly, for the subband b2, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M2a to be applied to the resource blocks r21 to r24. The PMI corresponding to the precoding matrix M2a is referred to as “PMI#2a” in FIG. 18. Further, for the subband b3, the mobile station feeds, back to the wireless station, a PMI corresponding to a precoding matrix M3a to be applied to the resource blocks r31 to r34. The PMI corresponding to the precoding matrix M3a is referred to as “PMI#3a” in FIG. 18.
In the case where the delay spread of the channel between the mobile station and the wireless base station is large and there is a large channel variation in the frequency direction, the variation in the precoding matrices of the resource blocks in each subband becomes large. According to FIG. 18, in each subband, the arrows of the precoding matrices point in random directions. This indicates that there is a large variation in the precoding matrices within each subband.
Under such a condition, there is a large dissimilarity between an ideal precoding matrix for each resource block in a subband and a precoding matrix indicated by a PMI which is selected by the mobile station for the subband and fed back to the wireless base station. Referring to, for example, the subband b1, it may be seen that the precoding matrix M1a of the subband b1 is largely dissimilar to each of the ideal precoding matrices m11a to m14a for the resource blocks r11 to r14. This means that one precoding matrix may not be a representative of the precoding matrices for the resource blocks r11 to r14. This is also the case with the remaining subbands b2 and b3. Accordingly, the wireless base station may not fully achieve the precoding effect because precoding is performed on a transmission signal of each resource block using a precoding matrix largely dissimilar to a corresponding ideal precoding matrix of the resource block. Accordingly, in the case described above, the precoding effect may not be achieved and thus the reception performance is degraded.
On the other hand, it may be considered of feeding back a PMI with respect to, not each subband, but each resource block. FIG. 19 illustrates PMI feedback control in which PMIs of all resource blocks are fed back. The mobile station feeds, back to the wireless base station, all of PMI#1b to PMI#12b which correspond to ideal precoding matrices m1b to m12b, respectively, of resource blocks r1b to r12b. However, such PMI feedback control with respect to each resource block is not a practical solution since the PMI feedback for all the resource blocks results in an increase in the amount of feedback information, which in turn reduces resources available for normal data transmission.